Optical system for improving the symmetry of the beam emitted from a broad area laser diode

ABSTRACT

An optical system is disclosed for improving the brightness symmetry of a beam emitted from a laser diode where the beam has a large width and a narrow height. The optical system includes a tilt plate for displacing one half of the width of the beam downwardly. A first beam steering prism functions to tilt the remaining, second half of the beam width in a plane parallel to the width dimension so that the second half travels towards the first half. A second beam steering prism is provided to tilt the second half of the beam so that its propagation axis is parallel to the propagation axis of the first half and wherein the second half is stacked above the first half. The optical system functions to improve the brightness symmetry of the beam by a factor of about five. The corrected beam can be used to improve the performance of a solid state laser which is end pumped by a broad area laser diode or a laser diode bar.

TECHNICAL FIELD

The subject invention relates to an optical system for improving thebrightness symmetry of the output beam generated by a broad area laserdiode. The optical system is particularly suited for improving systemefficiency when a broad area laser diode or laser diode bar is used toend pump a solid state laser.

BACKGROUND OF THE INVENTION

Over the last decade, there has been significant developments in thearea of solid state laser systems which are optically pumped using alaser diode. Laser diodes are desirable as pump sources since they arecompact and do not create the heating problems encountered with otherlight sources such as flashlamps. The light from the laser diode canalso be focused allowing for relatively efficient longitudinal or endpumping configurations.

There are also some drawbacks to using laser diodes. One significantproblem relates to the fact that laser diodes (and particularly highpower laser diodes) have an emitter region with a linear geometry andproduce an asymmetric output wherein the width is much greater than theheight. In a diode pumped laser system, maximum efficiency is achievedwhen the mode volume of the laser diode is matched well with thecirculating mode of the laser resonator. Ideally, the circulating modeof the beam is relatively symmetric so that a TEM₀₀ or fundamental modeoutput can be generated. Significant efforts are therefore required inorder to modify the highly asymmetric laser diode output beam in orderto match the symmetric resonator beam.

In practice, most efforts at modifying the asymmetric output of thelaser diode are directed towards improving the "spatial" symmetry of thebeam. Using various optical elements such as prisms and lenses, theaspect ratio of the beam can be modified. Unfortunately, spatialasymmetry is only part of the problem. More specifically, broad areaemitters also have severe "brightness" or "radiance" asymmetry.Brightness is defined as the number of photons per area per unit solidangle and is inversely proportional to the product of the area times thedivergence angle. In a typical high power laser diode, the divergence inthe narrow, height dimension is greater than the divergence in the widthdimension. However, since the width dimension is so much greater thanthe height dimension, the brightness in the height dimension can stillbe a hundred times greater than in the width dimension. In order tomaximize the pump efficiency, it would be desirable to minimize thebrightness asymmetry.

Unfortunately, these asymmetries in brightness are not modified with thebasic optics used to correct for spatial asymmetries. More specifically,when a basic optical system is used to reduce the width of the beam, thedivergence will proportionally increase. As can be appreciated, sincethe product of the width and divergence remains the same, brightnesswill not be effected. Therefore, there is a need for an improved opticalsystem which can not only modify the spatial asymmetries but modify thebrightness asymmetries as well so that higher power laser diodes can beused to pump lasers.

Accordingly, it is an object of the subject invention to provide anoptical system for improving the brightness symmetry of the output of abroad area laser diode or laser diode bar.

It is another object of the subject invention to provide an opticalsystem for improving the mode quality symmetry of the output of a broadarea laser diode.

It is a further object of the subject invention to provide an opticalsystem for improving the symmetry of the output of a broad area emitterso that it can be more efficiently coupled into the end of a symmetrictarget such as a gain medium or an optical fiber.

It is still another object of the subject invention to provide a diodelaser pumped solid state laser capable of generating a higher outputpower.

SUMMARY OF THE INVENTION

In accordance with these and other objects, the subject inventionprovides for an optical system for modifying the symmetry of a broadarea emitter or laser diode bar so that the brightness in the wide axiscan be substantially increased allowing for more efficient coupling oflight energy into a symmetric target. The optical system of the subjectinvention functions to divide the beam into two halves along the widthdimension. One half of the beam is displaced downwardly. The remaininghalf of the beam is then shifted over so that it stacked above the firsthalf and travels along a parallel propagation axis.

Using this approach, the total width of the beam is reduced by one halfwhile the height is doubled. Since the portions of the beam are merelyshifted (and not reshaped) the divergence will not change. Moreover,since the area in the width dimension is reduced by fifty percent whilethe divergence remains the same, the brightness in the width dimensionwill be doubled. The subject system functions to improve both thespatial and brightness symmetry of the beam such that the beam can bemore efficiently matched with a symmetric target such as the mode volumein an end pumped laser gain medium.

In the preferred embodiment, a tilt plate is aligned with a firstportion of the beam and functions to displace the first portion in adirection parallel to the height axis. A first beam steering prism isaligned with the remaining second portion of the beam and functions toangularly deviate the beam so that it is directed towards the firstportion. Once the second portion crosses over the first portion, asecond beam steering prism is utilized to angularly deviate the beam inthe opposite direction so that both beam portions will travel alongparallel propagation axes.

The subject optical system has been used to couple light from a broadarea emitter into a solid state laser resonator. This system has allowedthe output power of the laser to be doubled.

Further objects and advantages of the subject invention will becomeapparent from the following detailed description taken in conjunctionwith the drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view of an asymmetric beam generated by abroad area diode.

FIG. 2 is a perspective view of the optical system of the subjectinvention for improving the brightness symmetry of the asymmetric beamshown in FIG. 1.

FIG. 3 is top plan view of the tilt plate used to downwardly displace afirst portion of the beam.

FIG. 4 is side plan view of the tilt plate used to downwardly displace afirst portion of the beam.

FIG. 5 is top plan view of the prism pair used to shift a second portionof the beam over on top of the first portion of the beam.

FIG. 6 is side plan view of the prism pair used to shift a secondportion of the beam over on top of the first portion of the beam.

FIG. 7 is a cross-sectional view of the beam of FIG. 1 after passingthrough the optical system of the subject invention.

FIG. 8 is a perspective view of an alternate embodiment of the opticalsystem of the subject invention for improving the brightness symmetry ofan asymmetric beam.

FIG. 9 is a top plan view of the preferred embodiment of a diode pumpedsolid state laser using the optical system of the subject invention.

FIG. 10 is a cross-sectional view of the beam of FIG. 7 after passingthrough a second optical system of the subject invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, there is illustrating a cross-sectional view of theoutput beam 10 which is typically generated by a broad area laser diode.The ratio of the width to the height for a commercial four watt laserdiode is on the order of 500 to one. The divergence in the width axis isabout twelve degrees while the divergence in the height axis is aboutthirty degrees. This results in a brightness asymmetry of which exceeds100 to one. Output beams having even greater asymmetric configurationsare generated by laser diode bars.

This brightness asymmetries can be improved with the optical system 20of the subject invention. More specifically and as shown in FIGS. 2through 6, the subject optical system includes a first element 22 fordisplacing a portion of the beam in the downward direction. As best seenin FIGS. 3 and 4, element 22 is positioned to intercept the full heightof the beam 10 and a first portion 12 equal to about one-half of thebeam width. In the preferred embodiment, element 22 is defined by acubic tilt plate having opposed parallel faces 24 and 26. Faces 24 and26 should be provided with an antireflection coating. The refractionwhich occurs at the input face 24 deflects the beam portion 12downwardly. The refraction which occurs at the output face 26 reorientsthe beam portion 12 so that its propagation axis is parallel to, butdownwardly displaced from the input beam 10.

The subject invention further includes an optical means for shifting thesecond portion of the beam 14 over the first portion 12 such that bothportions of the beam travel along parallel propagation axes in a stackedconfiguration as shown in cross-section in FIG. 7. In the firstembodiment, the optical means for shifting the beam includes a pair ofbeam steering prisms 30 and 32. The first prism 30 is aligned with thesecond, remaining portion 14 of the width of the beam. The input face 34of prism 30 is aligned to be normal to the propagation axis of the beam.The output face 36 is formed at an oblique angle which causes the beamto refract in a plane parallel to the width axis as seen in FIGS. 2 and5. The angle of deviation can be between 20 and 40 degrees and ispreferably between 25 and 35 degrees.

The refraction of portion 14 of the beam causes the beam to traveltowards the first portion 12. Prism 32 is positioned at the spatiallocation where portion 14 crosses above portion 12. Prism 32 functionsto angularly deviate the beam 14 in the opposite direction so that thepropagation axis is parallel to the original direction of propagationand parallel to the propagation axis of portion 12.

Prism 32 is preferably identical in structure to prism 30. In addition,output face 36 of prism 30 and input face 42 of prism 32 are disposed atBrewster's angle with respect to the polarization state of the beam. Bythis arrangement, transmission is maximized without the need forantireflection coatings. Preferably, input face 34 of prism 30 andoutput face 44 of prism 32 are provided with antireflection coatings tomaximize transmission.

As seen in FIGS. 2 and 6, prism 32 must be positioned above beam portion12 so that little or no energy from portion 12 will be intercepted andredirected by the prism. To facilitate alignment, the elements should beconfigured to provide a small gap "G" between the upper and lowerportions 12 and 14. By this arrangement, scattering losses can beminimized.

The output of the optical system 20 is illustrated in FIG. 7. As can beseen, the two portions (12, 14) of the beam are stacked and haveparallel propagation axes. The total width of the beam has been reducedby fifty percent while the divergence has remained unchanged. For thisreason, the brightness in the width axis will have been doubled. Inaddition, the mode quality of the beam in this axis will also beimproved by a factor of two. Conversely, the height of the beam has beenmore than doubled which will reduce brightness in the height axis by atleast a factor of two. Thus, the subject optical system can produce atleast a four fold improvement in the brightness symmetry.

Although the use of a tilt plate and prism pair are believed to be apreferable approach to implementing the subject system, other variationsare possible. For example, beam portion 12 can be downwardly displacedby a pair of prisms rather than with a single tilt plate. It would alsobe possible to use a pair of mirrors to achieve the same result.Similarly, a pair of mirrors could be used to angularly deviate thesecond portion of the beam 14 in the place of prisms 30, 32.

Another alternative is illustrated in FIG. 8. As in the firstembodiment, a tilt plate 22 is used to downwardly deflect the firstportion 12 of the beam. Unlike the first embodiment, where a prism pairis used to steer the second beam portion 14, a tilt plate 50 performsthat function. Tilt plate 50 can have a similar configuration to thetilt plate 22. However, the input face 52 of tilt plate 50 is rotatedabout the height axis rather than about the width axis of the beam. Bythis arrangement, when the beam portion 14 enters the input face 52, itis refracted towards the output face 54 in a plane parallel to the widthaxis. When beam portion 14 passes through the output face, it isrefracted back so that its propagation axis is parallel to thepropagation axis of the first portion 12 and the two portions travel ina stacked, aligned configuration.

In this alternate embodiment, it is necessary that tilt plate 50 belocated downstream from the first tilt plate 22. As can be seen, theoutput face 54 of the tilt plate 50 must fall in the spatial regionwhere the first portion 12 of the beam is traveling prior to itsinteraction with the first tilt plate 22. Therefore the tilt platescould not be placed side by side in the manner shown for the tilt plate22 and prism 30 of the first embodiment.

FIG. 9 illustrates the use of the first embodiment of the optical system20 in a longitudinally pumped solid state laser 102 The resonator oflaser 102 is a unidirectional ring as described in detail in U.S. Pat.No. 5,052,815. The resonator assembly details are described in U.S. Pat.No. 5,170,409. The principle difference between the laser assemblyillustrated in U.S. Pat. No. 5,170,409 and the subject invention relatesto the use of the optical assembly 20 for improving the brightnesssymmetry of the beam. Lasers formed in accordance with the teachings ofthe above cited patents are marketed by the assignee herein, Coherent,Inc., under the trademark DPSS with a STAR resonator. The elements ofthe laser will be briefly described herein. Additional details can befound in the above cited patents.

As illustrated in FIG. 9, the laser resonator includes a first mirror104 which is transmissive to the 808 nm pump beam radiation from thelaser diode 106 and is reflective of the circulating radiation. Theresonator also includes a second mirror 108 which also serves as theoutput coupler. A solid state gain medium 112 is located within theresonator. In the commercial embodiment, the gain medium is formed fromNd:YAG. As described in U.S. Pat. No. 5,052,815, one face 114 of the YAGcrystal is formed with a refracting surface which allows a ringresonator to be formed with only two reflecting mirrors.

The resonator of laser 102 further includes a non-linear, frequencydoubling crystal 120. In the commercial embodiment, the crystal isformed from KTP. In this embodiment, both mirrors 104, 108 are coatedfor maximum reflectivity at the fundamental wavelength (1.06 microns).Mirror 108 is highly transmissive (about 95 percent) to the 532 nmradiation to maximize the output coupling of the frequency doubledradiation.

The gain medium 112 is pumped with the light from the laser diode 106.In the previous commercial embodiment, a two Watt broad area laser diodemanufactured by SDL was used to generate a rated output of 200 mw at 532nm. In an effort to increase this output, a more powerful pump laser wasutilized. In the preferred embodiment described herein, a four Wattbroad area laser diode manufactured by SDL (SDL Model No. 2382-P1) wasused. This diode laser had an expanded emitter area with a height ofapproximately one micron and a width of 500 microns. In order toefficiently couple the light from the higher power diode into theresonator, the optical system 20 of the subject invention was developed.

Similar to the prior art coupling arrangement, the light from the diode106 is first collimated by lens 130. Lens 130 has the effect of changingthe beam size to roughly two millimeters in height by three millimetersin width. Next the beam is anamorphically expanded by a pair of prisms140 and 142. After passing through this prism pair, the beam will stillhave a height of about 2 millimeters but the width will be expanded toabout 10 millimeters. The beam is expanded so that it can ultimately befocused to a smaller spot by lens 150.

The beam is then passed through the optical system 20 of the subjectinvention. As noted above, the tilt plate 22 functions to downwardlydisplace a first portion 12 of the beam 10. A suitable tilt plate can beformed from a 0.5 inch cube of BK-7 glass. The first beam steering prism30 functions to angularly displace a second portion 14 of the beam inplane parallel to the width axis of the beam. In the preferredembodiment, the displacement angle is 30.5 degrees. Complementary beamsteering prism 32 functions to displace the beam portion 14 once againso that its propagation axis is parallel to the propagation axis of beamportion 12. The stacked portions of the beam (as illustrated in FIG. 7)propagate to a lens 150. Lens 150 functions to focus the beam into themode volume of the laser resonator in the gain medium.

In order to maximize the output power of the laser, and take advantageof the higher input power, it is important that the transmissioncharacteristics of the optical system 20 be relatively high (at leasteighty percent.) One method of achieving this goal is to insure that theelements are arranged to reduce scattering loss. In the preferredembodiment, prism 30 and tilt plate 22 are mounted together. A thinlayer of glue is used to attach the elements together so that thejunction is narrow and transmission losses are minimized.

Interestingly, it was noted that the output emission pattern of the SDLdiode used in the experiments had a centrally located brightness minimumextending in the width dimension. The physical origin of the minima is a50 micron void located in the center of the 500 micron laser diodeemission area. By placing the mechanical union between the tilt plateand the prism at this minima, the beam scattering loss can be furtherreduced.

The placement of the optical system 20 downstream from the anamorphicprisms 140 and 142 also helps to reduce scattering loss. Morespecifically, since the prisms expand the beam in the width dimension,less light energy is available at the interface between the tilt plateand the prism 30. It should be noted that the optical system could alsobe positioned between the two anamorphic prisms, however, someadditional scattering loss would be expected.

Another approach to reducing beam scattering at the interface would beto modify the lay-out shown in FIG. 9 by inserting a plate 160 (shown inphantom line in FIG. 9) in front of the optical system 20. Plate 160 canbe disposed at Brewster's angle and would function to displace one halfof the beam laterally (parallel to width axis) in a manner similar totilt plate 50 (FIG. 8). The two beam halves would then be spatiallyseparated and would not have to intersect the connection between thetilt plate 22 and the prism 30.

As noted above, it is also desirable to control the propagation paths sothat a gap. "G" (FIG. 7) is created between the beams. The size of thegap G is controlled by the angle of the tilt plate 22. In the preferredembodiment, the tilt plate is set to an angle of incidence of 38 degreeswith respect to incoming beam and the gap "G" is on the order of 1.4millimeters. Since the beam has a gaussian intensity distribution and inreality does not have the sharp edges illustrated in the Figures, thisgap prevents the upper edge of the lower portion 12 of the beam fromintercepting the lower edge of prism 32.

One additional benefit of providing the gap "G" is that height dimensionof the beam cross-section is increased which further improves thebrightness symmetry. In the preferred embodiment, the width of the beamportions (as shown in FIG. 7) is on the order of five millimeters. Theheight of each of the individual portion is about two millimeters, sothe total height of the beam (including the 1.4 millimeter gap g) isabout 5.4 millimeters.

Based on the dimensions given above, the improvement in brightnesssymmetry can be calculated. More specifically, the brightness of thelight emitted from the diode 106 in the height dimension is more than100 times greater than the brightness in the width dimension. Afterpassing through the optical system 20 of the subject invention, thebrightness in the width dimension will be increased by a factor of two,while the brightness in the height dimension will be reduced by a factorof 2.5 so that the brightness symmetry will have been improved by afactor of five.

It should be noted that there are other methods for increasing the pumppower into the laser cavity. One prior art approach is to use two diodelasers and overlap the output beams. Unfortunately, in order to overlapthe propagation axes of two beams relatively complex and expensivepolarization multiplexing schemes must be used. In contrast, using thetechniques herein, an improved result can be achieved with simple, lowcost optical elements. In fact, the total cost of the three opticalelements is under eighty dollars.

Another advantage of the subject system is that the transmission lossesare quite low. In experiments it has been determined that thetransmission loss created by the tilt plate and two prisms is only aboutthree percent. The total transmission loss of all of the opticalelements from the laser diode 106 to the focusing lens 150 is on theorder of ten percent. Thus, a very high percentage of the lightgenerated by the laser diode may be focused into the gain medium.

In experiments utilizing the subject optical system 10, a four Wattlaser diode was used to pump a laser resonator as described above. Anoutput power of 550 milliwatts at 532 nm was generated.

As noted above, the subject optical system can improve the brightnessasymmetry by a factor of five. Even with this improvement, thebrightness in the height dimension is still significantly brighter thanin the width dimension. If further improvements are desired, it would bepossible to place one or more additional optical systems 20 in the beampath in a cascaded fashion. The optical elements of the second system(shown as a phantom block at 20a in FIG. 9) would have a height at leastequal to the combined height of the two beam portions (12, 14) and awidth equal to one half of the five millimeter beam width. When placedin the path of the stacked beam portions 12 and 14, the resultant outputwould be produce four stacked beam segments in a pattern as shown inFIG. 10. The portions 12a and 14a derive from the right halves of thesegments 12 and 14 of FIG. 7, while the portions 12b and 14b derive fromthe left halves of segments 12 and 14 of FIG. 7. As compared to the beamshown in FIG. 7, the width of the segments would be reduced by fiftypercent and the total height of all the segments would be increased bymore than fifty percent. Using one additional optical system 20a, thebrightness symmetry of the beam could be improved by a another factor offive.

Although in the illustrated embodiments, the optical elements areconfigured to divide the width of the beam in half, other geometriescould be used. Variations could be particularly useful where multipleoptical systems 20 are used in a cascaded fashion. In this case,geometries could be employed which would allow the shape of the beam tobe modified, such as by making the beam more round.

As noted above, the subject optical system 20 may be useful inapplications other than longitudinally pumping a solid state gainmedium. For example, similar considerations are relevant when attemptsare made to launch light into the aperture of an optical fiber. Thus,the subject optical system could be used to improve the brightnessasymmetry in a device where energy from a laser diode is to be coupledto an optical fiber for remote delivery.

It should be noted that terms such as downward, over and below are usedin the specification merely to simplify the discussion and are notintended to limit the subject invention.

While the subject invention has been described with reference to thepreferred embodiments, various changes and modifications could be madetherein, by one skilled in the art, without varying from the scope andspirit of the subject invention as defined by the appended claims.

We claim:
 1. A laser comprising:an optical resonator; a solid state gainmedium located within the resonator; a light source for opticallypumping the gain medium, said light source being defined by a laserdiode generating a light beam having a cross-section wherein the widthis greater than the height; means for optically coupling the beam fromthe light source into the gain medium, said coupling means includingfirst optical means aligned with and intercepting one half of the beamin the width dimension, said first optical means functioning to displacesaid one half of the beam downwardly; and second optical means alignedwith and intercepting the other half of the beam in the width dimension,said second optical means for shifting the other half of the width ofthe beam over said one half in a manner such that both halves of thebeam travel along parallel propagation axes in a stacked configurationthereby improving the brightness symmetry of the beam.
 2. A laser asrecited in claim 1 wherein said coupling means further includes a meansfor collimating the beam emitted from the laser diode.
 3. A laser asrecited in claim 2 wherein said coupling means further includes a meansfor anamorphically expanding the beam.
 4. A laser as recited in claim 1wherein said coupling means functions to end pump the gain medium.
 5. Alaser as recited in claim 1 wherein said coupling means includes a lensfor focusing the beam into the gain medium.
 6. A laser as recited inclaim 1 wherein said first optical means is a tilt plate.
 7. A laser asrecited in claim 6 wherein said second optical means is a tilt plate. 8.A laser as recited in claim 6 wherein said second optical means includesa first prism for bending the propagation axis of the beam so that saidother half passes above said one half and a second prism aligned withand intercepting said other half of the beam for redirecting said otherhalf such that both halves of the beam will travel along parallelpropagation axes in a stacked configuration.
 9. A laser as recited inclaim 1 wherein the beam halves are spatially separated to avoid lossesdue to scattering.
 10. A laser as recited in claim 1 further including asecond coupling means located between said first coupling means and saidgain medium for further enhancing the brightness symmetry of the beam.11. A laser as recited in claim 1 further including an optical elementaligned with the first portion of the beam and positioned between thelaser diode and the coupling means, said optical element functioning todisplace one half of the beam from the other half in a plane parallel tothe width axis to minimize scattering losses.
 12. A laser as recited inclaim 1 wherein said coupling means transmits at least eighty percent ofthe beam.